Light detection and ranging (LIDAR) scanners are non-contact measurement devices that work by emitting a very narrow light pulse and analyzing the reflection of the light pulse from an object. In order to scan a three dimensional (3D) area from a single observation position, these scanners typically employ actuators to rotate the LIDAR assembly and/or use rotating mirrors to reflect the light pulse around the scene. The result is a 3D point cloud of precise distance measurements of the scene from the observation position of the LIDAR scanner. The device can also map color information from the scene by integrating imaging sensors with the LIDAR ranging sensors.
In order to expand the area covered by the LIDAR scan, the scanner is moved to another observation position (also referred to as the next vantage position) and a new scan is generated. Scans from various vantage positions can be merged to create a complete 3D map of the area. The vantage positions must be selected to fill in areas of occlusions (i.e., areas hidden by object shadows) and efficiently cover large areas with minimum scan points
The 3D LIDAR scanners maintain constant speed on rotary actuators to distribute the points evenly about a polar coordinate system. Considering (r,θ,φ) as the polar coordinate system relative to the scanning position, constant actuation speed and sampling of the scanners distributes the scan points evenly along the θ and φ coordinates. However, the range variation will depend upon the relative positioning of the objects in the scene and varies from scene to scene. This range variation produces a non-uniform point distribution of points in the Cartesian coordinates about the same scanner position reference. As a result, objects closer to the LIDAR scanner will have a higher point density compared to objects further from the scanner.
Also, the laser beam emanating from the scanner diverges from the source and creates a spot on the object being scanned. Objects further away from the scanner have larger spot diameters compared to the objects that are close by. When the larger spots hit corners of the objects the precision of the measurement is adversely affected. In order to control both the resolution and distribution of the points of the scan, and provide scan data that are uniformly distributed within a scene, adaptive control is needed for both the speed of the rotary mechanisms and divergence of the beam.
Different aspects of a representative conventional LIDAR scanner are illustrated in FIGS. 1-4. The LIDAR scanner 10 includes a laser diode (not shown) used for emitting an infrared light pulse. The light pulse 12 is reflected from the object 14 as a reflected pulse 16 which is received by a detector (not shown) in the scanner 10. Rotary mirrors and actuators (not shown) are used for steering the light pulse 12 within an area of interest of the object 14. Electronics (not shown) are used for processing and calculating the precise 3D coordinates of the scanned object based on the calculated travel time of the emitted and reflected pulses 12, 16 and the angles of displacement of the actuators and rotary mirror.
As illustrated in FIGS. 2-4, the conventional 3D scanner 10 may rotate from side-to-side in opposite angular directions, represented by double-arrow 18, about a vertical axis 20 and/or up-and down in opposite angular directions, represented by double-arrow 22, about a horizontal axis 24. Referring to FIG. 3, rotation of the 3D scanner 10 about the vertical axis 20, without rotation about the horizontal axis 24, results in a 2D distribution of emitted pulses, represented by the straight dotted lines 26, in a horizontal plane. Although not depicted in the figures, rotation of the 3D scanner 10 about the horizontal axis 24, without rotation about the vertical axis 20, would result in a 2D distribution of emitted pulses in a vertical plane. Referring to FIG. 4, rotation of the 3D scanner about both the vertical axis 20 and the horizontal axis 24 results in a distribution of emitted pulses through a 3D space, represented by the straight dotted lines 26 and the curved dotted lines 28 extending out of the plane of the straight dotted lines 26.
With conventional 3D scanners, the rotary actuators are operated at a constant speed during a scan. These scanners produce non-uniform point distributions within a 3D scan scene as measurement points closer to the scanner will have higher point densities than points further from the scanner as illustrated in the FIG. 5. More specifically, it can be seen that for the same angular rotation theta (θ), objects closer to the scanner (distance R1) will have a higher point density (i.e., a smaller point distribution spacing D1) compared to objects further from the scanner (distance R2) while having a lower point density (i.e., a larger point distribution spacing D2). FIG. 6 depicts a graph of measurement distances, in meters versus point distribution spacing, in meters for various θ values.
Conventional LIDAR scanners also have fixed laser beam divergence. FIG. 7 shows components used in the typical 3D LIDAR scanner 10. A lens 28 is placed in the light transmission path to diverge the beam 30. The divergence of beam 30 is necessary so that a finite spot size (e.g. beam spot diameter 32) is created and the beam 30 does not miss small features in the objects 34, 36 being scanned. However, for surfaces located far away from the scanner but within the scanner range, the beam divergence leads to large spot diameters 32 which cause a noisy return signal (not shown), also known as “edge noise”, at the edges 38 of the object 34. The range data produced from the scanner will be erroneous in such situations.
FIG. 7 also shows the typical LIDAR returns and best estimates for the location of the surfaces of two walls (i.e., objects 34, 36), which walls are depicted in FIG. 8. The scene of FIG. 8 has been reconstructed from data reproduced during a LIDAR scan. While both walls 34, 36 were scanned with uniform scan patterns, the laser spot hit both the surfaces at the corner 38. This leads to erroneous reconstruction of the object geometry, resulting in the corner 38 being obscured. Varying the beam divergence to a smaller angle γ (referenced on FIG. 7) enhances the precision of the scan. Using a smaller beam diameter for the entire scene, however, is very inefficient. This inefficiency is due to the fact that more measurements are required for covering an area with a small spot diameter beam and edge noise only occurs when the spot is split by two surfaces at different distances from the scanner.
LIDAR scans taken from a single point also include areas that are occluded by objects in the scene. The scanner must be moved to a variety of positions to fill in data points representing the occluded areas. It is also necessary to move and position the LIDAR scanner to enlarge the coverage area or field of view of the scan. Under conventional methods, a human operator assesses the scene to be scanned and determines the next best scan position. Such positioning is prone to errors and may still result in a scene map having gaps in the scan data.